1. Field of the Invention
The present invention relates to an ink transfer medium of the electrically fusible type, in which an electric signal is converted into thermal energy to transfer an imaging ink to an ink reception material.
2. Prior Art
One type of prior art ink transfer medium is disclosed in the Japanese Patent Application (OPI) No. 84735/78 (the term "OPI" as used herein means an "unexamined published application"). It comprises an ink film coated with a low-melting-point ink on one side, and a thermal printing head pushed onto the other side of the ink film to conduct heat to melt the ink to transfer it for imaging. Since the heat is conducted through a relatively long distance in the medium, the speed of printing of the ink is low, taking 1 msec or more per dot to perform the printing. Further, since transmissible energy in the medium is low, the choice of materials for the ink is greatly constrained, thus lowering the controllability of the transference of the ink and making dot modulation impossible; or the main constituent of the ink is limited to be a wax or the like.
A second prior art ink transfer medium is disclosed in the Journal of the Institute of Image Electronics Engineers of Japan, No. 1, Vol. 11, 1982 and the Drafts 17 for the 12-th National Convention of the Institute. In this second prior art medium, electric signals corresponding to an image are applied from stylus electrodes into the ink layer of the medium through the ink carrier thereof to generate heat to melt the ink layer to transfer it for imaging. As shown in FIG. 5, the ink transfer medium comprises an anisotropically electrically conductive layer 51, an electrically conductive heating layer 52 and the electrically conductive ink layer 53. The anisotropically electrically conductive layer 51 is the carrier of the layers 52 and 53, and is composed of a resin and a metal powder dispersed therein. The layer 51 is formed as a ribbon. The layer 51 may be substituted by an electrically conductive film of high electric resistance.
A problem with this second kind of prior art recording medium is that the electrical conductivity of the ink transfer medium makes it difficult to control the tones of colors, thereby making it difficult to make a color image through the use of the medium. Further, the ink carrier of the medium has large electrical energy dissipation and is relatively low in mechanical strength. The accuracy of dots printed through the use of the medium is low. Since the electrical anisotropy of the medium is insufficient, current spreading occurs in the carrier ribbon of the medium to cause a large loss of energy.
A third type of prior art ink transfer medium is disclosed in the Japanese Patent Application (OPI) No. 93585/81. It is composed of carrier of moderate electric resistance, a heating layer, and return passage electrodes. Applied electrical current passages are produced in the medium through the carrier thereof by stylus electrodes to generate heat to melt an ink layer to transfer it for imaging. The carrier of the ink transfer medium has no electrical conductivity anisotropy, however, causing the area of each dot printed through the use of the medium to be enlarged. Since a spreading current which does not contribute to the effective localized generation of the heat in the medium is excessive, the energy efficiency of the medium is low. Since the carrier of the medium is electrically resistive, the contact resistance between the carrier and the stylus electrode is high.
A forth type of prior art ink transfer medium is shown in FIG. 6. Return passage electrodes 62 are provided on the same side as printing electrodes 61, as shown in FIG. 6, and electric signals corresponding to an image are applied from stylus electrodes so that electrical current passages 67 extending to the return passage electrodes are produced in the heating resistor layer 63 of the medium comprising the heating resistor layer, an electrically conductive layer 64 and an ink layer 65, to generate heat in the heating resistor layer to melt the ink layer to transfer it for imaging.
In this forth type of prior art device, since an applied electric current flows through the heating resistor layer of the ink transfer medium twice due to the electrical current passage extending to the return passage electrode, a double energy loss is caused. Since sliding contact is performed twice by the stylus electrode and the return passage electrode for the medium, a double heat loss is caused due to the contact resistance between the electrodes. Since some electric resistance needs to be provided in the electrically conductive passage of the ink transfer medium in order to cause an electrical current to flow, with priority, to the return passage electrode, a large heating loss is caused in the electrically conductive passage.